Today, you can reference an instance property as a member of its type to access it as a fully unbound function, which is currently curried:
struct Foo {
var x: Int
func foo(y y: Int) { return x + y }
}
let foo = Foo.foo
foo(Foo(x: 1))(y: 2) // returns 3
However, this is problematic for `mutating` methods. Since the first argument is `inout`, the mutation window for the parameter formally ends before the second argument can be applied to complete the call. Currently we miscompile this, and form a closure over a dangling pointer, leading to undefined behavior:
struct Bar {
var x = 0
mutating func bar() { x += 1 }
}
let bar = Bar.bar
var a = Bar()
bar(&a)() // This might appear to work, if we don't optimize too hard
let closure: () -> ()
do {
var b = Bar()
closure = bar(&b)
}
closure() // This scribbles dead stack space
var c = Bar() {
didSet { print("c was set") }
}
bar(&c)() // This will scribble over c after didSet is called, if not worse
We can close this hole by disallowing a reference to Bar.bar, like we already disallow partial applications. However, I think it would be in line with our other simplifications of the function type system to change the type of `Bar.bar` and other unapplied instance method references to no longer be curried. In addition to providing a model for unapplied instance methods that works with mutating methods, this would also eliminate a type difference between free functions and methods of the same arity, allowing for easier code reuse. For instance, `reduce` takes a closure of type (State, Element) -> State. Flattening the formal type of instance methods would allow binary methods to be used as-is with `reduce`, like binary free functions can:
func sumOfInts(ints: [Int]) -> Int {
return ints.reduce(0, combine: +)
}
func unionOfSets<T>(sets: [Set<T>]) -> Set<T> {
return ints.reduce([], combine: Set.union)
}
What do you all think?
-Joe
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